Rock bolt assembly and procedure for two-stage grouting operations



Aug. 22%. 1967 c. l. WILLIAMS ,7

ROCK BOLT ASSEMBLY AND PROCEDURE FOR TWO-STAGE GROUTING OPERATIONS 2 Sheets-Sheet 1 Filed Jan. 11, 1965 INVENTOR. CHESTER W/LL Mill 75 Sheets-Sheet 2 22. 19 c. 1. WILLIAMS ROCK BOLT ASSEMBLY AND PROCEDURE FOR TWO-STAGE GROUTING OPERATIONS Filed Jan. 11, 1965 2 w W w W m 2 m 7 a. v N W. 'MI W PVC P n 7 k .W l N. n w 4 M Q 5 M a I. E v, fl in. L H B n. mm c ma 6 H n 5 2 4 -v n i n": u 5 4 a m J? W in: 0 MY rwr-mmmw mm nu w ii 4 1 Inn 4 9 4 V k i 1 b United States Patent 3,336,758 ROCK BOLT ASSEMBLY AND PROCEDURE FOR TWO-STAGE GROUTING OPERATIONS Chester I. Williams, 347 Greenbriar SE., Grand Rapids, Mich. 49506 Filed Jan. 11, 1965, Ser. No. 424,656 12 Claims. (Cl. 61-35) ABSTRACT OF THE DISCLOSURE A method of grouting a rock bolt in position, in which a preliminary charge of grout traverses the entire bolt rod, and is permitted to set, a second charge being delivered via an opening in a coupling interconnecting inner'and outer bolt rod sections.

This invention relates to the construction and use of rock bolts. The installation of these bolts in dense rock formations will normally involve installing the bolt in the drilled hole in the rock, followed by securely setting an anchor device mounted at the inner end of the bolt. The bolt rod is then-pre-stressed by application of heavy torsion to a'nut against a surface bearing plate so that the rod is placed under a degree of tension approximating its working load. The rod is then preferably grouted in this condition by pumping a quantity of cementitious liquid into the space within the hole surrounding the rock bolt. When this material has hardened, it increases the bond between the bolt and the rock formation for the transfer of stresses, and also'provides a protective shield around the bolt assembly. The pre-stressing operation is very important, as the function of the bolt is then applied to the rock without the necessity of displacement of the rock before the bolt becomes useful.

In particularly soft rock formations, it is difiicult or impossible to set the anchor device tightly enough to develop the necessary pre-stress in the bolt rod. The softness of the rock does not provide sufiicient resistance against the expansion forces of the anchor device, with the result that pr'e-stress possibilities in soft rock have been severely limited. Applicant has provided a procedure and a rock bolt assembly which permits pre-stress to be developed in rock bolts installed in very soft rock through the use of a two-stage grouting operation. The rock bolt is inserted in the hole in the rock formation, and an initial charge of grout is injected which submerges the anchor device and a small inner portion of the length of the bolt rod. This charge of grout is permitted to harden, and the resulting engagement of the rock bolt assembly is sutficiently improved to permit the pre-stressing operation to be performed. A second charge of grout may be injected after the pre-stressing operation, or the pre-stress may be performed while the secondary charge of grout is still plastic. A sufficient elongation in the bolt rod results from-the pre-stressing operation so that it should not be done after the grout has hardened. To do so will result in breaking the bond between the rod and the grout, and disturbing the sealing elfect of the grout as well.

In order to provide for the secondary injection of the grout, it becomes necessary to establish two different paths through which the grout can flow. The first of these must necessarily traverse the full length of the bolt rod assem'bly, or ,substantially so. The second charge of grout 3,336,758 Patented Aug. 22, 1967 must follow another path, however, since the hardened grout will block the further flow through the full initial path, unless the charge is forced out into the surrounding rock formation beyond the rock bolt assembly. To grout the outer remainder of the bolt rod, it becomes necessary to provide a lateral passageway at an intermediate point along the length of the bolt rod, with this passageway controllable by some sort of valve that can be maintained in closed condition during the initial grouting. The details of this invention that provide this feature will be analyzed through a discussion of the particular embodiments illustrated in the accompanying drawings. In the drawings:

FIGURE 1 presents a sectional elevation showing a rock bolt assembly installed in a hole in a rock formation, prior to the injection of grout.

FIGURE 2 illustrates the condition of the inner section of the rock bolt after the initial charge of grout has been injected, and the outer section of the rock bolt (including the coupling) has been removed.

FIGURE 3 illustrates the re-coupling of the sections of the rock bolt into the condition shown in FIGURE 1, but with the lateral passage open to receive the second charge of grout, and showing the second charge of grout in position.

FIGURE 4 is a section on the plane 4-4 of FIGURE 1, on an enlarged scale. v

FIGURE 5 is a section on the plane 5-5 of FIG- URE 3.

FIGURE 6 is a sectional elevation of a modified form of the invention, in condition corresponding to the injection of the initial charge of grout.

FIGURE 7 is a view of the modification shown in FIGURE 6, with the outer rod section back-rotated to expose one of the lateral discharge ports of the coupling.

FIGURE 8 is a further modification of the invention in condition preparatory to receiving the first charge of grout.

FIGURE 9 is a further modification of the invention, in the initial condition.

FIGURE 10 is a section on the plane 1010 of FIG- URE 9.

The rock bolt assembly generally indicated at 10 in FIGURES 1-3 is shown installed in the hole 11 in the rock formation 12. The rock bolt assembly includes the outer rod section 13, the inner rod section \14, the coupling 15, and an anchor device generally indicated at 16. Both the inner and outer sections of 13 and 14 are tubular, as is the coupling 15. The stresses transferred by the fully installed rock bolt assembly are taken primarily by the bearing plate 17 resting against the surface 18 of the rock formation, with this loading being transferred by the nut 19 and the beveled washer 20.

- The structural details of the coupling 15 are shown in FIGURES 4 and 5. The unit is tubular in configuration, and preferably threaded throughout its length. The ends are countersunk as shown at 21 and 22 to facilitate the engagement with the ends of the rod sections 13 and 14. A screw plug 23 is engageable with the lateral hole 24 to provide a passage for the flow of grout during-the secondary grouting operation. The removable plug 23 serves the function of a valve and is removed during the time in which the outer rod section 13 (together with the coupling 15) are withdrawn from the bore in the rock formation, as indicated in FIGURE 2. A set screw is located where it will normally engage the end 13 of the outer rod section to assure that the coupling 15 remains rotatably fixed with respect to the outer rod section, rather than remaining on the inner rod section 14. The controllable lateral discharge port provided by the opening 24 and the plug valve 23 makes it possible to deliver an initial charge of grout shown at 26 in FIG- URE 2. After this has set, the outer section of the rod, together with the coupling, may be withdrawn so that the plug 23 may be removed. When the unit is re-connected as shown in FIGURE 3, the secondary charge of grout 27 can be delivered through the opening 24 to occupy the space within the hole 11 around the outer rod section 13.

The modified form of the invention shown in FIGURE 6 includes the outer and inner rod sections 28 and 29, respectively, joined by the coupling 30. In all of these forms of the invention, it is preferable that the threaded length of the ends of the rod sections engaging the coupling is approximately half of the length of the coupling so that the coupling will be properly centered upon the threaded ends, with the same engagement of threads on both rod sections. The coupling is provided with a pair of ports 31 and 32, approximately equally spaced from the adjoining ends of the rod sections 28 and 29. The rod sections 28 and 29 are tubular, and the condition of the device shown in FIGURE 6 corresponds to FIG- URE 1, in the sense that the unit is ready for the delivery of the initial charge of grout. After this has been accomplished, the outer rod section may be back-rotated, with the result that either the opening 31 or the opening 32 (or a portion of both of these) will be exposed. The ends of the rod sections 28 and 29 thus perform the function of valves in place of the removeable plug 23, without the necessity of removing the outer rod section 28 and the coupling 30 from the hole for adjustment.

After the outer rod section has been back-rotated enough to induce a movement of about a half inch axially either with respect to the coupling 30, or with respect to the inner rod section 29, the outer rod section may be cleared of grout either with water under pressure, or by compressed air. Whichever is the case, the outflow will take place through the opening 31 or 32 (or partially through both), and the initial charge of grout may be permitted to set. The rock bolt assembly may then be prestressed, followed by a secondary injection of grout which will then fiow out through the holes 31 and/ or 32. The length of the coupling 30 should be established such that there will be adequate thread engagement between the end 28 or 29 and the coupling for full transfer of stress when in the backed-off condition of the assembly.

The rubber ring 33 is added to the inner rod section 29 to facilitate grouting when the hole in the rock formation extends upwardly from the point at which the rock bolt assembly is inserted. The ring 33 will act as a dam bearing lightly against the wall of the hole in the rock, tending to confine the initial charge of grout so that it will properly surround the inner section 29 and the anchor device.

Referring to FIGURE 8, a modified form of the invention is illustrated in which the outer rod section 34 is connected to the inner rod section 35 by the coupling 36. Spaced side holes 37 and 38 serve the same function as the holes 31 and 32 of FIGURES 6 and 7. A pair of balls shown at 39 and 40 are provided as abutments transferring the axial forces between the ends of the rod sections 34 and 35 to minimize the jamming action resulting from the heavy torque applied to set the anchor device. With this a-rrangement, the outer rod section may be back-rotated with a minimum danger of unscrewing the inner rod section 35 from the anchor device. A pair of balls 39 and 40 are used in preference to a single ball to minimize the danger that a single ball might assume a position on the axis of the rod sections 34 and 35 to function as a ball valve, and thus block the fiow of grout to the inner rod section. To prevent this same ball-valve effect from blocking the outlet holes 37 and 38, these holes should be of slightly greater diameter than that of the balls 39 and 40.

Referring to FIGURES 9 and 10, a further modification of the invention illustrates the provision of a single ball 41 between the ends 42 and 43 of the outer and inner rod sections, respectively. The coupling 44 is similar to the couplings 30 and 36, and is provided with the pair of holes 45 and 46. To prevent the single ball 41 from obstructing the central opening 47 in the inner rod section 43, a strip of leaf-spring material 48 is inserted in a position to maintain a biasing effect holding the ball 41 in an eccentric position. Both the balls 41 and 3940 should be of hardened steel in order to sustain a tremendous end pressure resulting from the setting of the anchor device.

The particular embodiments of the present invention which have been illustrated and discussed herein are for illustrative purposes only and are not to be considered as a limitation upon the scope of the appended claims. In these claims, it is my intent to claim the entire invention disclosed herein, except as I am limited by the prior art.

I claim:

1. A method of grouting a rock bolt in two stages, comprising:

assembling a tubular bolt rod including at least two sections connected by a tubular coupling provided with a hole at an intermediate point along its length, the inner of said sections having an anchor device secured thereto;

inserting said assembled bolt rod into a hole in a ground formation with said coupling hole blocked;

injecting a preliminary charge of grout into said hole via both inner and outer sections of said bolt rod, clearing said outer section of said grout, and permitting said inner charge of grout to set:

opening said coupling hole, and injecting a second charge of grout via said outer section and said coupling hole.

2. A method as defined in claim 1, wherein said coupling hole is blocked with a screw plug, and said outer rod section and coupling are removed from said hole in the ground formation for removal of said plug prior to injecting said second charge of grout.

3. A method as defined in claim 1, wherein the end of at least one of said rod sections is initially disposed over said coupling hole to block the same, and said section is partially unscrewed from said coupling to permit flow through said coupling hole prior to said second injection of grout.

4. A rock bolt assembly for two-stage grouting, comprising:

a tubular bolt rod having at least inner and outer separate sections;

a tubular coupling having threaded engagement with said rod sections, said coupling having at least one hole in the wall thereof at an intermediate point along the length of said coupling;

an anchor device secured to the inner extremity of said inner rod section; and

valve means for closing and opening said coupling hole.

5. An assembly as defined in claim 4, wherein said inner rod section is provided with a ring of flexible material as a dam for obstructing flow of grout along the outside of said bolt rod.

6. An assembly as defined in claim 4, wherein said valve means is a screw plug.

7. An assembly as defined in claim 4, wherein said valve means is the end portion of at least one of said rod sections.

8. An assembly as defined in claim 4. wherein said 5 coupling has two holes, and said assembly includes means for centering the ends of said rod sections in said coupling at a position between said holes.

9. An assembly as defined in claim 4, wherein at least one ball is interposed between the ends of said rod sections in said coupling.

10. An assembly as defined in claim 9, including means for holding said ball in an eccentric position.

11. An assembly as defined in claim 9, including a plurality of balls.

12. An assembly as defined in claim 11, wherein said balls are less in diameter than said coupling hole.

6 References Cited UNITED STATES PATENTS 5/1948 Beijl 61-45 X 4/1958 Dempsey 6145 FOREIGN PATENTS 1962 Australia. 1,268,624 1961 France.

790,483 1958 Great Britain. 

1. A METHOD OF GROUTING A ROCK BOLT IN TWO STAGES, COMPRISING: ASSEMBLING A TUBULAR BOLT ROD INCLUDING AT LEAST TWO SECTIONS CONNECTED BY A A TUBULAR COUPLING PROVIDED WITH A HOLE AT AN INTERMEDIATE POINT COUPLING PROVIDED THE INNER OF SAID SECTIONS HAVING AN ANCHOR DEVICE SECURED THERETO: INSERTING SAID ASSEMBLED BOLT ROD INTO A HOLE IN A GROUND FORMATION WITH SAID COUPLING HOLE BLOCKED; INJECTING A PRELIMINARY CHARGE OF GROUT INTO SAID HOLE VIA BOTH INNER AND OUTER SECTIONS OF SAID BOLT ROD, CLEARING SAID OUTER SECTION OF SAID GROUT, AND PERMITTING SAID INNER CHARGE OF GROUT TO SET; OPENING SAID COUPLING HOLE, AND INJECTING A SECOND CHARGE OF GROUT VIA SAID OUTER SECTION AND SAID COUPLING HOLE. 